The investigators identified the two masterminds as Pham Thi Giang, 34, and Nguyen Thi Hong, 44.

They caught the couple was they were transporting two girls, aged 16 and 17, on a bus to Tan Thanh border gate in Lang Son Province on June 11.

Giang and Hong, both Hanoi locals, told the police that the girls, who live in nearby Hoa Binh Province, agreed to follow them to the capital city after they lied to them about well-paying jobs at clothing stores there.

Their statements led the police to arrest Nguyen Tien Thinh, 66, and Do Van Hoan, 32, whose job was to walk the victims across the border and sell them to buyers.

Human traffickers arrested on June 11 by Hanoi police, who also rescued three teenage girls from being sold to Chinese brothels. Photo credit: VietNamNet

The four traffickers confessed to preying on teenage girls from poor families in rural and mountainous areas, whom they typically sold for between VND20 and VND25 million each (around US$1,000).

Police say they have confessed to selling five victims since last September.

Figures released in 2013 showed that Vietnam police busted more than 3,000 human-trafficking attempts along its border with China since 2003, rescuing around 1,200 would-be victims in 2012, alone.